The present invention relates to a centrifugal compressor.
In a centrifugal compressor, centrifugal action produced by rotation of the impeller feeds fluid at a high speed into a diffuser passage. The fluid fed to the diffuser passage is decelerated in the diffuser passage to be increased in pressure. The fluid at the increased pressure is thereafter fed into a volute like a scroll provided on the outer circumference of the diffuser passage.
The passage cross-sectional area of the diffuser passage is set to allow fluid to be fed into the diffuser passage at a desired maximum flow rate. Thus, if the fluid is fed into the diffuser passage at a small flow rate and the volute (on the downstream side of the diffuser passage) is in high pressure, the fluid flows backward to cause surging. The occurrence of surging hampers stable operation of the centrifugal compressor.
In this regard, a compressor disclosed in Japanese Laid-Open Utility Model Publication No. 6-63897 includes a throttle portion 102 for adjustment, specifically for control of the passage cross-sectional area of a diffuser passage 101 as shown in FIG. 10. The throttle portion 102 includes a disk-shaped diffuser plate 103 forming one side area of the diffuser passage 101. The diffuser plate 103 is provided in a recess 105 formed in a housing 104 having the diffuser passage 101 to be capable of reciprocating.
The diffuser plate 103 is coupled to one end of each of multiple rods 106 spaced at equal intervals in the circumferential direction. The opposite end of each rod 106 is coupled to a piston 108 arranged in a cylinder 107 of the housing 104 in a manner that allows the piston 108 to reciprocate. The inside of the cylinder 107 is partitioned by the piston 108 into a head chamber 109 and a rod chamber 110. The head chamber 109 is connected via a communication passage 111 to a discharge portion 112. The rod chamber 110 is connected via a communication passage 113 to a suction portion 114. The rod chamber 110 houses a spring 115 that biases the diffuser plate 103 in a direction that increases the passage cross-sectional area of the diffuser passage 101.
If fluid is fed into the diffuser passage 101 at a small flow rate and the discharge portion 112 is in high pressure, large differential pressure is generated between the suction portion 114 and the discharge portion 112. This makes the pressure in the head chamber 109 overcome the biasing force of the spring 115 to move the piston 108 in a direction that increases the volume of the head chamber 109. Thus, the diffuser plate 103 moves in a direction that reduces the passage cross-sectional area of the diffuser passage 101. This reduces the passage cross-sectional area of the diffuser passage 101 to cause the fluid to flow through the diffuser passage 101 smoothly.
In contrast, the lower the pressure at the discharge portion 112, the greater the flow rate of the fluid delivered into the diffuser passage 101 becomes. In this case, the differential pressure between the suction portion 114 and the discharge portion 112 is small. Thus, the biasing force of the spring 115 moves the piston 108 in a direction that reduces the volume of the head chamber 109. Meanwhile, the diffuser plate 103 moves in the direction that increases the passage cross-sectional area of the diffuser passage 101. This increases the passage cross-sectional area of the diffuser passage 101 to cause the fluid to flow through the diffuser passage 101 smoothly.
In the compressor described in the aforementioned publication, the passage cross-sectional area of the diffuser passage 101 is reduced by reducing the passage cross-sectional area between the diffuser plate 103 and the wall surface of the housing 104 facing the diffuser plate 103. Thus, failing to accurately maintain the diffuser plate 103 in its position makes it impossible to keep the diffuser passage 101 in a constant narrowed condition.